1. Field of the Invention
This invention relates generally to the construction of electronic device assemblies which employ surface mount packages, and more particularly to a method for substantially improving the shock resistance of such electronic assemblies.
2. Description of the Related Art
To build sophisticated electronics, such as computers, stereos, medical instrumentation, and more, discrete electronic components, herein referred to as electronic devices, are mounted onto circuit boards. The circuit boards provide a rigid support for the electronic components, and may additionally provide the interconnections between the electronic devices. As used herein, the term xe2x80x9celectronic device assemblyxe2x80x9d refers to a circuit board having one or more electronic devices mounted on its surface.
Surface mount technology (SMT) is a construction technique for electronic device assemblies in which the terminals of electronic devices are attached to the surface of a printed circuit board (PCB),by solder or some other conductive adhesive. In SMT the device terminals each have a flat (planar) contact surface that rests on corresponding conductive xe2x80x9clanding padsxe2x80x9d on the PCB surface. SMT may be distinguished from other construction techniques which generally employ xe2x80x9cthrough pinxe2x80x9d terminals on their electronic device packages. In these other construction techniques, the device terminals are pins which are placed in holes passing through the circuit board and sealed there by solder or some other conductive adhesive.
SMT fabrication permits components to be mounted to both sides of the PCB. As such, a primary advantage which SMT provides over xe2x80x9cthrough pinxe2x80x9d construction techniques is the increased packing density, i.e. the number of components on the PCB per unit of area, which may be achieved by mounting electronic devices on both sides of the PCB. A primary disadvantage of SMT is the decreased reliability relative to the xe2x80x9cthrough pinxe2x80x9d construction techniques. In the xe2x80x9cthrough pinxe2x80x9d techniques, the terminal physically passes through a hole in the board, thereby providing a strong, shock resistant mechanical coupling to the board. In SMT, the terminals are physically coupled to the board only by conductive adhesive. The terminals, and perhaps even the entire device, may separate from the PCB when the terminal and PCB are subjected to a force that exceed the mounting strength of the adhesive. The mounting strength is often measured by determining the minimum force needed to dislodge a terminal from the landing pad. High reliability and a high packing density are both essential for certain electronic device assemblies such as implantable pacemakers and other implantable medical devices. At least one approach exists for improving the reliability of SMT device assemblies. Referring now to FIG. 1, an electronic device package 102 is shown having leads 104, 106 coupled to a PCB by adhesive joints 108, 110. The mechanical coupling between electronic device package 102 and the PCB is further strengthened by a glob 112 of a non-conductive adhesive.
Electronic device package 102 and leads 104, 106 are illustratively typical of those used for capacitors, resistors, and diodes. These and other two- and three-terminal devices tend to have more mass per terminal than most ICs and other electronic devices with more than three terminals. Consequently, when a PCB is accidentally dropped onto a hard surface or in some other manner subjected to excessive shock forces, the adhesive joints of these devices are subjected to higher stresses than other devices with less mass per terminal. To reduce the likelihood of joint failure, additional adhesive is often applied as one or more globs (such as glob 112 in FIG. 1) between the individual packages and the PCB to provide mechanical reinforcement. This technique requires an additional step during construction of the device assembly for applying the additional adhesive, and also requires that the device packages be spaced apart more than is strictly necessary. The additional space is required to provide the device assemblers with the necessary physical and visual access for applying the non-conductive adhesive globs.
A better method for improving the reliability of electronic device assemblies while increasing the packing density and which does not require additional steps during construction of the device assemblies is desirable.
Accordingly, there is provided herein a method for increasing the mounting strength of surface mount device packages. In one embodiment, an electronic device is made up of a device package and one or more electrically conductive terminals. For surface mounting, the device terminals are each provided with a mounting surface which couples to a corresponding contact pad on a circuit board. To improve the device""s mounting strength, the mounting surfaces are provided with at least one groove. When conductive adhesive is used to mount the device a circuit board, this groove serves to form the conductive adhesive into a ridge or xe2x80x9cdamxe2x80x9d over the contact pad. Measurements show that the presence of this ridge enhances the mounting strength by at least an order of magnitude.
The present invention includes an electronic device assembly with improved reliability and a method for constructing such an assembly. The assembly comprises a circuit board, an electronic device, and conductive adhesive The electronic device has one or more terminals which are configured to be mounted by conductive adhesive on corresponding contact pads on the circuit board. At least one of the terminals is provided with a groove which is configured to form at least some of the conductive adhesive into a ridge over the contact pad.
One contemplated method for constructing such an electronic device assembly comprises: (1) forming a conductive adhesive layer on one or more contact pads on a circuit board; and (2) placing one or more corresponding grooved terminals of an electronic device onto the conductive adhesive to form at least some of the conductive adhesive into a ridge over the contact pad. Certain adhesives may additionally need to be heated and cooled to permanently mount the electronic device on the circuit board.
Another contemplated method for constructing such device assembly comprises: (1) coating a grooved surface of one or more grooved terminals of an electronic device with an electrically conductive adhesive; and (2) placing the coated grooved surface of the one or more terminals against corresponding contact pads on a circuit board. As before, certain adhesives may additionally need to be heated and cooled to mount the electronic device on the circuit board.